Chromatography is essentially a physical method of separation in which constituents of a test sample in a carrier gas or liquid are adsorbed or absorbed and then desorbed by a stationary phase material in a column. A pulse of the sample is introduced into a steady flow of carrier gas, which carries the sample into a chromatographic column. The inside of the column is lined with a liquid, and interactions between this liquid and the various elements of the sample—which differ based upon differences among distribution coefficients of the elements—cause the sample to be separated into the respective elements. At the end of the column, the individual components are more or less separated in time. Detection of the gas provides a time-scaled pattern, typically called a chromatogram that by calibration or comparison with known samples, indicates the constituents of the test sample. An example of the process by which this occurs is described in U.S. Pat. No. 5,545,252 to Hinshaw.
Often, the sample is first obtained using a sampling device, which subsequently transfers the sample to the chromatograph. One means of obtaining a sample and introducing it into a chromatographic column is known as “headspace sampling.” In conventional headspace sampling, sample material is sealed in a vial and subjected to constant temperature conditions for a specified time. Analyte concentrations in the vial gas phase should reach equilibrium with the liquid and/or solid phases during this thermostatting time. The vial is subsequently pressurized with carrier gas to a level greater than the “natural” internal pressure resulting from thermostatting and equilibration. Then the pressurized vial is connected to the chromatographic column in such a way as to allow for the transfer of a portion of the vial gas phase into the column for a short period of time. An example of such a sampling device is disclosed in U.S. Pat. No. 4,484,483 to Riegger et. al. An example of a chromatographic system employing such a sampling device is disclosed in U.S. Pat. No. 5,711,786 to Hinshaw, which describes using a chromatographic injector between the vial and the chromatographic column.
Typically, it is desired to pre-concentrate the analytes in the sample, and occasionally, remove moisture therefrom, prior to introducing the sample into the chromatographic column. Accordingly, as disclosed in U.S. Pat. Nos. 5,792,423 and 6,395,560 to Markelov, these systems will typically include some kind of “trap” for this purpose, which retains the analytes as they are carried through the trap, and which are later released from the trap, usually by heating, and swept into the chromatographic column.
Various types of traps have been suggested to perform this pre-concentration (and possible moisture removal) prior to introducing the sample into a chromatographic column. Often, it is advantageous to use an adsorbent trap of some sort to adsorb the analytes, which can later be desorbed, as opposed to an absorbent because, since the anhydrous substance absorbs water, repeated use of the anhydrous substance is likely to be limited and require frequent replacement. Accordingly, numerous arrangements employing an adsorbent trap have been employed for the purpose of pre-concentrating the analytes of a sample extracted by a sampling device such as a headspace sampler. Examples of such arrangements are disclosed in U.S. Pat. No. 5,932,482 to Markelov and U.S. Pat. No. 6,652,625 to Tipler.
However, to date, these systems have resulted in a number of disadvantages. First, in order to accomplish this multiple stage process of extracting a sample fluid, transferring it to the trap, trapping it and untrapping it, and transferring it to the chromatographic column, these systems have employed complex assemblies of parts and/or valves situated in the flow path of the fluid containing the analytes to be measured. These extra devices and valves not only increase cost and space, but increase dead-volume areas and surface active sites. This results in sample dispersion, dilution, or loss, and causes excessive peak broadening on the chromatogram. Another disadvantage present in some of these systems is the unidirectional path of flow for both adsorption and desorption, inhibiting the ability to first trap heavier compounds and then more volatile compounds by using multiple adsorbents.
What is desired, therefore, is a system for interfacing a sampling device and a chromatograph and for pre-concentrating analytes in a sample prior to introducing the sample into the chromatographic column that is inexpensive to manufacture and does not take up a lot of space. What is further desired is a system for interfacing a sampling device and a chromatograph and for pre-concentrating analytes in a sample prior to introducing the sample into the chromatographic column that reduces the amount of dead volume areas and surface active sites. What is also desired is a system for interfacing a sampling device and a chromatograph and for pre-concentrating analytes in a sample prior to introducing the sample into the chromatographic column that facilitates the use of multiple adsorbents.